Among methods for treating exhaust gas in which NOx is removed from exhaust gas in a reduction denitration unit and then SO2 is removed in a wet desulfurization unit using an alkaline absorbing solution as an absorbent, there has been researched a method for treating metallic mercury and a mercury compound (hereinafter, collectively referred to as mercury, unless otherwise stated) in exhaust gas while performing denitration and desulfurization at the same time.
Mercury in flue gas exists in forms of metallic mercury which are insoluble in water, and mercury chloride which is soluble in water. When in the form of metallic mercury, mercury is difficult to dissolve in water. When mercury is in the metallic form, the efficiency of removing mercury by a wet desulfurization unit is decreased. Conversely, when mercury is in the form of HgCl or HgCl2, HgCl or HgCl2 in exhaust gas it may be dissolved in water through the gas-liquid contact in the wet desulfurization unit, and thereby mercury can be removed. In other words, if metallic mercury can be converted into mercury chloride in the presence of a catalyst such as a denitration catalyst, mercury can be removed in the desulfurization unit located downstream.
An example of such a conventional method for treating exhaust gas utilizing this scheme will be described with reference to FIG. 3. In FIG. 3, a NH3 supply spot 20 and a supply spot 21 are provided in a flow path from a boiler 10 to a reduction denitration unit 60. At the NH3 supply spot 20, NH3 supplied from a NH3 tank 30 is injected into exhaust gas. At the supply spot 21, a mercury-chlorinating agent such as HCl is injected into the exhaust gas from a tank 40 for supplying the mercury-chlorinating agent. The exhaust gas from the boiler 10 is introduced into the reduction denitration unit 60. In the reduction denitration unit 60, NH3 and NOx in the exhaust gas into which NH3 and HCl are injected react with each other, and simultaneously metallic Hg is oxidized to HgCl2 in the presence of HCl. After passing through an air heater 70 and a heat collector 80, the soot and dust are removed in a dust collector 90. Then, SO2 and HgCl2 in the exhaust gas are simultaneously removed in a wet desulfurization unit 100. At this point, an excessive amount of HCl is contained in the exhaust gas having passed through the reduction denitration unit 60, but is never discharged from a stack, since HCl is absorbed by an alkaline aqueous solution such as lime milk in the desulfurization unit 100. Together with the above-described method, a system in which a chlorinating agent such as HCl is sprayed upstream of a denitration catalyst to oxidize (chlorinate) mercury on the catalyst, and then the mercury is removed in a wet desulfurization unit located downstream thereof (see, for example, Patent Literature 1) has been proposed.    Patent Literature 1: JP 10-230137 A